1. Field of the Invention
The present invention relates to an image display device, and more particularly to a liquid crystal display having a touch panel function with a built-in photo-sensor.
2. Description of the Related Art
JP-A-58-66142 (FIGS. 2 and 3) discloses a liquid crystal display with a built-in photo-sensor, especially a liquid crystal display capable of displaying the image and perform a reading operation from the photo-sensor at the same time. In this liquid crystal display, a photo-sensor (pixel sensor) is disposed at each pixel in a display matrix (hereinafter referred to as a “display part”), and a read signal line for photo-sensor is disposed separately from a source wiring for image display and connected with a detector configured to detect a threshold for detecting turning ON/OFF of the photo-sensor. Also, JP-A-4-222018 discloses a liquid crystal display with a built-in photo-sensor in which switching means switches between the image display and the coordinate detection in time division to make the image display and the coordinate detection by reading the photo-sensor consistent. Further, JP-A-2006-323642 discloses a display device having a selector part for reading the photo-sensor in time division is also well known.
However, in the pixel sensor circuit as described above, since a sensor is disposed corresponding to each of a plurality of signal lines, the number of signal lines is increased, and proportionally the number of sensors is also increased, as the display screen has larger size, higher definition and higher resolution, which makes the area larger and requires the higher cost.
As a method for solving the above-mentioned problem, a selector containing a thin film transistor (hereinafter referred to as “TFT”) connected to the read signal line of the photo-sensor is disposed within the display device, the detector is connected via each selector to every plural read signal lines. Accordingly, the number of sensors is reduced by time division method, and the size of photo-sensor detection circuit itself is reduced, which suppresses the cost. If this method is adopted, there is an adverse influence on the potential at the detection timing before the end of detection period due to influence of coupling noise on the read signal line caused by polarity reversal of the source wiring potential after the gate wiring is turned ON, whereby it is difficult to detect the threshold for detecting turning ON/OFF of the photo-sensor, which may results in a deterioration of the precision of the coordinate detection is degraded.